Today computer-based systems can be found most everywhere and have made inroads into many devices that are used by society everyday, such as cell phones, handheld computers, automobiles, medical devices, personal computers and the like. In general, society has placed much reliance on computer-based systems to handle everyday tasks such as simple tasks like balancing checkbooks to relatively complex tasks such as predicting the weather. As technology improves, more and more tasks are migrated to computer-based systems. This, in turn, causes society to become more and more reliant on these systems.
A typical computer-based system comprises a system board and optionally one or more peripheral devices, such as display units and disk units. The system board often contains one or more processors, a memory subsystem and other circuitry, such as serial device interfaces, network device controllers and hard disk controllers.
The type of processors that are employed on a particular system board usually depends on the type of tasks performed by the system. For example, a system that performs a limited set of tasks, such as monitor emissions generated by an automobile engine and adjust an air/fuel mixture to ensure the engine is burning fuel completely may employ a simple specialized processor that is tailored to performing these tasks. On the other hand, a system that performs many different tasks, such as managing many users and running many different applications, may employ one or more complex processors that are general purpose in nature, configured to perform high-speed calculations and manipulate data to minimize the response time to servicing the users' requests.
The memory subsystem is a storage that holds information (e.g., instructions, data values) used by the processors. The memory subsystem typically comprises controller circuitry and one or more memory devices. The controller circuitry is usually configured to interface the memory devices with the processors and enable the processors to store and retrieve information to and from the memory devices. The memory devices hold the actual information.
Like the processors, the type of devices employed in a memory subsystem is often driven by the type of tasks performed by the computer system. For example, a computer system may have the task of having to boot without the assistance of a disk drive and execute a set of software routines that do not change often. Here, the memory subsystem may employ non-volatile devices, such as flash memory devices, to store the software routines. Other computer systems may execute very complex tasks that require a large high-speed data store to hold large portions of information. Here, the memory subsystem may employ high-speed high-density Dynamic Random Access Memory (DRAM) devices to store the information.
Demand for flash memory devices has continued to grow significantly because these devices are well suited in various embedded applications that require non-volatile storage. For example, flash is widely used in various consumer devices, such as digital cameras, cell phones, USB flash drives and portable music players, to store data used by these devices. Market demand for flash memory has led to tremendous improvements in flash memory technology over the past several years both in terms of speed and density. These improvements have led to the prediction that flash memory-based devices may one day replace hard disk drives in applications that continue to use disk drives for mass storage.
Some flash devices employ serial interfaces which are used to perform operations, such as read, write and erase operations, on memory contained in the devices. These operations are typically selected on a device using command strings that are serially fed to the devices. The command strings typically contain a command that represents the operation to be selected as well as parameters. For example, a write operation may be selected by serially feeding an information string to the device that contains a write command, the data to be written and an address in the memory where the data is to be written.